Thousands of striking truck drivers protested Tuesday amid heavy police presence in central Johannesburg as labor unrest continued across South Africa, leading to fears of renewed violence.

The truckers marched to the nearby area of Braamfontein to submit a petition to the transport bargaining council for a 12 percent pay raise, said the South African Transport and Allied Workers' Union, which organized protests around South Africa.

Vincent Masoga, a spokesman for the union, said it wanted a "massive attendance" of 15,000 people in Johannesburg who were expected to remain peaceful throughout the march. The protesters, some wielding sticks or placards and others dancing or singing freedom chants, gathered in Johannesburg's Beyers Naude Square for the march.

Labor unrest is also affecting other industries in South Africa, where strikes have spread to several gold, platinum and chrome mines, damaging investor confidence in a country that is among the world's top producers of valuable minerals.

Strikes began in August at Lonmin's Marikana mines, where police shot dead 34 striking workers on Aug. 16, a level of state violence not seen since the end of apartheid in 1994. A retired judge is currently leading an official inquest into the Marikana incident and related violence that killed at least 44 people. The commission of inquiry will determine the roles played by the police, Lonmin and two mining unions, and whether any of those under investigation could have done something to prevent the violence.

In September the Marikana strikers returned to work after accepting a pay increase of up to 22 percent through negotiations that also involved church leaders as mediators. But two of South Africa's most powerful unions said in a joint statement Tuesday that the Marikana settlement set a bad precedent for labor relations in South Africa.

"Lonmin should have known that getting wage negotiations to be facilitated by the churches and allowing everybody, no matter their legal status, to play a role in the negotiations will create precedents that they will not be willing to repeat anywhere else," the Congress of South African Trade Unions and the National Union of Mineworkers said in a statement.

The unions said that wage demands can still be resolved through collective bargaining rather than "contemplating taking short cuts such as mass dismissals of workers..."

Even as South Africa grapples with the Marikana incident, there appears to be no end in sight to the ongoing labor unrest. A truck traveling on a highway in Cape Town was pelted with stones Tuesday, causing it to overturn, and two other trucks were set on fire, according to the South African Press Agency.

The National Union of Mineworkers said Monday that one of its officials had been taken into intensive care after a petrol-bomb attack on his house. The union said the victim is its top official at Anglo American Platinum's Khomanani branch and that the attack was carried out by people who are deliberately intimidating union members.

Workers have been on strike for weeks at Anglo American Platinum, the world's largest platinum producer. Similar strikes are ongoing at Gold Fields, Samancor Chrome Western Mine and Anglo Gold Ashanti, a top gold producer whose operations have been brought to a standstill.